This update adds some initial Storybook stories for the Icon and IconButton
component.

Most of the stories were derived from the component's README.md documentation.
Additional stories were added to showcase prop variation (e.g. `Icon` size),
as well as composition (e.g. `IconButton` grouping).

In order to render certain UI for the stories, some simple stateless
functional components were created in the `.stories.js` file.